Current industry practice for low-visibility landings is dependent on airport ground equipment and inertial navigation equipment. These techniques are limited to landings at those runways which are equipped with highly reliable transmitters of radio frequency localizer and glide slope information. These existing systems either land the aircraft using an automatic pilot or aid the pilot in landing the aircraft by providing the pilot with autopilot control commands displayed on a Head Up Display (HUD).
It has been suggested that future systems make use of information received from the Global Positioning System (GPS) in conjunction with on-board Inertial Navigation systems (INS) to generate the necessary precise navigation for landing. However, in addition to the external satellites required for GPS, these systems are currently envisioned to require ground stations at known locations near the runway for the differential precision necessary for landing. Other proposed systems provide the pilot with a real time image of the runway scene as derived from millimeter wave (MMW), X-Band, or infrared (IR) frequencies.
The following are further examples of navigation systems known in the art.
U.S. Pat. No. 5,136,297 to Lux et al discloses an autonomous landing system. The Lux patent includes a navigation unit employed in the system which includes a sensor, flight position data, an image correction unit, a segmentation unit, a feature extraction unit and a comparison unit. The Lux patent discloses that a comparison is conducted as to whether or not a sequence of features in the overflight path image pattern agrees with features found in a reference store, such as map data which is stored in the system. Further, Lux discloses the use of a radar navigation system for use as a sensor in the system.
U.S. Pat. No. 4,698,635 to Hilton et al discloses a radar guidance system coupled to an inertial navigation apparatus. The system includes a master processor, a radar altimeter, a video processor, a memory and a clock. The memory has stored therein cartographic map data.
U.S. Pat. No. 4,495,580 to Keearns is cited to show a navigation system including a radar terrain sensor and a reference map storage device for storing data representing a terrain elevation map.
U.S. Pat. No. 4,910,674 to Lerche discloses a navigation method which includes a correlator for comparing terrain reference data with processed altitude data obtained with a wave sensor.
U.S. Pat. No. 4,914,734 to Love et al is cited to show a map-matching aircraft navigation system which provides navigational updates to an aircraft by correlating sensed map data with stored reference map data.
U.S. Pat. No. 4,891,762 to Chotiros is cited to show a pattern recognition system for use in an autonomous navigation system.
The above-mentioned prior systems suffer from one or more of the following problems:
1) Reliance on ground-based systems for precise terminal landing information severely reduces the number of runways available for Cat III a and b landings (currently 38 runways in the U.S.).
2) Reliance on GPS and differential ground transmitters for GPS creates a need for currently rare ground equipment and a lack of reliability (based on the military nature of GPS). The GPS is a military program owned, operated, and paid for by the United States Air Force originally intended for military navigational purposes and is designed so that civilian use can be made of it but at a reduced accuracy. The military uses a very special code which gives them better accuracy, that is called the P code. The normal civilian code is called the C code which is good to about 30 m in accuracy; however, the military retains the right to disable the C code to the point where the accuracy goes down to about no better 100 m. This is what the military refers to as "selective availability" so that in time of conflict they can turn on selective availability and deny the enemy the ability to navigate better than 100 m. There are a number of schemes for getting around the inaccuracies imposed by the military. However, the Air Force has maintained a position that they are against any of these schemes which improve the accuracy when they are trying to make it inaccurate.
The lack of reliability is also a result of the fact that, in order to be accurate, at least four satellites must be present in the overhead view; and, if one of the four satellites fails, then the accuracy will be degraded. Thus, the reliability is not just based on the on-board equipment, i.e., the GPS receiver, but it is also based on the reliability of the satellites themselves.
3) Additional sensors, such as MMW and IR, currently envisioned for systems to provide pilots with the "situational awareness" necessary to successfully land in low visibility conditions are expensive additions to the on-board flight equipment and are marginal in performance. MMW real-beam radars provide "grainy" low resolution images which are difficult to interpret and IR systems cannot penetrate in many types of fog that cause the "low visibility" in the first place.